Epidermal Langerhans cells (LC) express many properties characteristic f macrophages and play a pivotal role in the development of allergic contact hypersensitivity and, possibly, skin allograft rejection. The recent availability of reproducible histologic, electron microscopic, and immunologic techniques for the identification of LC, the ability to isolate and maintain viable LC, and the establishment of a continuous line of cells retaining LC characteristics enable our laboratory to carry out a series of experiments dealing with: 1. Production of biologically active products by LC and the identification of factors affecting their production. 2. Maturation of a continuous line of LC. Insights into possible therapeutic regimens to prevent or suppress contact hypersensitivity manifested as allergic contact dermatitis, may be gained with the identification of LC products and activators. Successful induction of LC granule synthesis could allow for studies of the morphogenesis and isolation of LC granules.